bblayers.conf.sample: update a comment

Update the comment on ONIRO_LAYERS_CONF_VERSION to tell the user
that they also need to change ONIRO_REQUIRED_LAYERS_CONF_VERSION
and where to find it. Cover both cases of the sample and the
local file.

Signed-off-by: Marta Rybczynska <marta.rybczynska@huawei.com>